1. Field of the Invention
The present invention relates to switch-type oscillating circuits, and more particularly, to a switch-type oscillating circuit in which switch circuits selectively switch one of two oscillating circuits to an operating state and sufficient isolation is provided between the two oscillating circuits when the output ends of the two oscillating circuits are coupled with the input end of a common circuit through a coupling circuit.
2. Description of the Related Art
As mobile communication systems used in the world, there are a digital cellular system (DCS) used in Great Britain, Germany, Italy, France, and a part of Asian nations, and a global system for mobile communications (GSM), which was adopted in 1982 as the European integrated standard method for digital portable telephones, used in Europe, the United States of America, Africa, and a part of Asian nations.
In the DCS, a local-station frequency is assigned to 1805 MHz to 1880 MHz, a mobile-station frequency is assigned to 1710 MHz to 1785 MHz, the oscillation frequency of a voltage-controlled oscillating circuit (VCO) in a portable telephone is assigned to the 1700 MHz band, the number of used channels is 374, and the Gaussian minimum shift keying (GMSK) modulation method is used. In the GSM, a local-station frequency is assigned to 925 MHz to 960 MHz, a mobile-station frequency is assigned to 880 MHz to 915 MHz, the oscillation frequency of a voltage-controlled oscillating circuit (VCO) in a portable telephone is assigned to the 900 MHz band, the number of used channels is 124, and the Gaussian minimum shift keying (GMSK) modulation method is used.
Since these two mobile communication systems, the DCS and the GSM, are essentially different in method, it is necessary to use two portable telephones, one for mobile communication in the DCS and the other for mobile communication in the GSM, to subscribe to the two mobile communication systems.
Because the DCS and the GSM use the GMSK modulation method, as described above, and only differ in assigned frequency, a portable telephone which can be used both in the DCS and the GSM has already been proposed, provided with a switch-type oscillating circuit which includes two voltage-controlled oscillating circuits, a first voltage-controlled oscillating circuit for oscillating at a frequency of the 1700 MHz band and a second voltage-controlled oscillating circuit for oscillating at a frequency of the 900 MHz band, and switch circuits; and in which the two voltage-controlled oscillating circuits are selectively used under the control of the switch circuits.
When this portable telephone, which can be used both in the DCS and the GSM, is used for mobile communication in the DCS, the switch circuits switch the first voltage-controlled oscillating circuit to an operating state and the second voltage-controlled oscillating circuit to a non-operating state so as to obtain an oscillation output from the first voltage-controlled oscillating circuit. When the portable telephone is used for mobile communication in the GSM, the switch circuits switch the first voltage-controlled oscillating circuit to a non-operating state and the second voltage-controlled oscillating circuit to an operating state so as to obtain an oscillation output from the second voltage-controlled oscillating circuit.
FIG. 3 is a circuit diagram showing an example structure of a known switch-type oscillating circuit in a portable telephone which can be used both in the DCS and the GSM.
As shown in FIG. 3, the known switch-type oscillating circuit is provided with a first voltage-controlled oscillating circuit 31 for oscillating at a frequency of the 1700 MHz band, a second voltage-controlled oscillating circuit 32 for oscillating at a frequency of the 900 MHz band, a coupling circuit 33, a common circuit 34, a first-oscillation-signal output terminal 35, a second-oscillation-signal output terminal 36, a common input terminal 37, a signal output terminal 38, a first-switch-signal supply terminal 39, a second-switch-signal supply terminal 40, a power terminal 41, a first-frequency-setting-voltage supply terminal 42, and a second-frequency-setting-voltage supply terminal 43.
The first voltage-controlled oscillating circuit 31 is formed of a first oscillation transistor 31.sub.1, a first switching transistor 31.sub.2, an emitter load resistor 31.sub.3, an isolation inductor 31.sub.4, a varactor diode 31.sub.5, an inductor 31.sub.6, a bypass capacitor 31.sub.7, resonant capacitors 31.sub.8 and 31.sub.10, a resonant inductor 31.sub.9, base bias resistors 31.sub.11, and 31.sub.12, feedback capacitors 31.sub.13 and 31.sub.14, a series resistor 31.sub.15, and a grounding capacitor 31.sub.16. These circuit elements 31.sub.1, to 31.sub.16 are connected as shown in FIG. 3.
The first switching transistor 31.sub.2 constitutes a first switch circuit, goes on when a first switch signal sent to the first-switch-signal supply terminal 39 has a positive polarity (high level), and goes off when the first switch signal has a ground potential (low level). The varactor diode 31.sub.5, the resonant inductor 31.sub.9, and the resonant capacitors 31.sub.8 and 31.sub.10, constitute a first resonant circuit for specifying the first oscillation frequency of the first voltage-controlled oscillating circuit 31. The resonant frequency of the first resonant circuit is set to a predetermined frequency in the 1700 MHz band by changing a first frequency setting voltage sent to the varactor diode 31.sub.5 through the first-frequency-setting-voltage supply terminal 42.
The second voltage-controlled oscillating circuit 32 has almost the same structure as the first voltage-controlled oscillating circuit 31, and is formed of a second oscillation transistor 32.sub.1, a second switching transistor 32.sub.2, an emitter load resistor 32.sub.3, an isolation inductor 32.sub.4, a varactor diode 32.sub.5, an inductor 32.sub.6, a bypass capacitor 32.sub.7, resonant capacitors 32.sub.8 and 32.sub.10, a resonant inductor 32.sub.9, base bias resistors 32.sub.11 and 32.sub.12, feedback capacitors 32.sub.13 and 32.sub.14, a series resistor 32.sub.15, and a grounding capacitor 32.sub.16. These circuit elements 32.sub.1 to 32.sub.16 are connected as shown in FIG. 3.
The second switching transistor 32.sub.2 constitutes a second switch circuit, goes on when a second switch signal sent to the second-switch-signal supply terminal 40 has a positive polarity (high level), and goes off when the second switch signal has a ground potential (low level). The varactor diode 32.sub.5, the resonant inductor 32.sub.9, and the resonant capacitor 32.sub.8 and 32.sub.10, constitute a second resonant circuit for specifying the second oscillation frequency of the second voltage-controlled oscillating circuit 32. The resonant frequency of the second resonant circuit is set to a predetermined frequency in the 900 MHz band by changing a second frequency setting voltage sent to the varactor diode 32.sub.5 through the second-frequency-setting-voltage supply terminal 43.
The coupling circuit 33 is formed of a first coupling capacitor 33.sub.1, an inductor 33.sub.2, a capacitor 33.sub.3, a second coupling capacitor 33.sub.4, an inductor 33.sub.5, and a capacitor 33.sub.6. These circuit elements 33.sub.1 to 33.sub.6 are connected as shown in FIG. 3. The inductor 33.sub.2 and the capacitor 33.sub.3 form a first trap circuit which resonates at the second oscillation frequency of the second voltage-controlled oscillating circuit 32. The inductor 33.sub.5 and the capacitor 33.sub.3 form a second trap circuit which resonates at the first oscillation frequency of the first voltage-controlled oscillating circuit 31. The coupling circuit 33 provides sufficient isolation between the first voltage-controlled oscillating circuit 31 and the second voltage-controlled oscillating circuit 32 since the first trap circuit and the second trap circuit are connected as shown in FIG. 3.
The common circuit 34 is formed of an amplifying transistor 34.sub.1, a collector load inductor 34.sub.2, an emitter resistor 34.sub.3, a grounding capacitor 34.sub.4, base bias resistors 34.sub.5 and 34.sub.6, impedance-matching capacitors 34.sub.7 and 34.sub.8, and a bypass capacitor 34.sub.9. These circuit elements 34.sub.1 to 34.sub.9 are connected as shown in FIG. 3.
The switch-type oscillating circuit having the above structure operates in the following way.
When the portable telephone is used for mobile communication in the DCS, a control section (not shown) sends a first switch signal having the positive polarity (high level) to the first-switch-signal supply terminal 39 and a second switch signal having the ground potential (low level) to the second-switch-signal supply terminal 40 to turn on the first switching transistor 31.sub.2 and to turn off the second switching transistor 32.sub.2. In the first voltage-controlled oscillating circuit 31, the ground end of the emitter load 31.sub.3 of the first oscillation transistor 31.sub.1 is connected to the ground through the collector-emitter path of the first switching transistor 31.sub.2, which is on, an operation current flows between the collector and the emitter of the first oscillation transistor 31.sub.1, and a normal oscillation operation is achieved. On the other hand, in the second voltage-controlled oscillating circuit 32, the ground end of the emitter load 32.sub.3 of the second oscillation transistor 32.sub.1 is not connected to the ground because the second switching transistor 32.sub.2 is off. Therefore, an operation current does not flow between the collector and the emitter of the second oscillation transistor 32.sub.1, and an oscillation operation is not achieved.
An oscillation signal (at a frequency in the 1700 MHz band) obtained by the first oscillation transistor 31.sub.1 is sent from the emitter thereof to the coupling circuit 33 through the first-oscillation-signal output terminal 35, and then is sent to the common input terminal 37 of the common circuit 34 through the first coupling capacitor 33.sub.1, and the first trap circuit formed of the elements 33.sub.2 and 33.sub.3 in the coupling circuit 33. Then, the oscillation signal is amplified by the amplifying transistor 34.sub.1 in the common circuit 34, and the amplified oscillation signal is sent through the signal output terminal 38 to a circuit (not shown) which uses the oscillation signal.
When the portable telephone is used for mobile communication in the GSM, the control section sends a first switch signal having the ground potential (low level) to the first-switch-signal supply terminal 39 and a second switch signal having the positive polarity (high level) to the second-switch-signal supply terminal 40 to turn off the first switching transistor 31.sub.2 and to turn on the second switching transistor 32.sub.2. In the second voltage-controlled oscillating circuit 32, the ground end of the emitter load 32.sub.3 of the second oscillation transistor 32.sub.1 is connected to the ground through the collector-emitter path of the second switching transistor 31.sub.2, which is on, an operation current flows between the collector and the emitter of the second oscillation transistor 32.sub.1, and a normal oscillation operation is achieved. On the other hand, in the first voltage-controlled oscillating circuit 31, the ground end of the emitter load 31.sub.3 of the first oscillation transistor 31.sub.1 is not connected to the ground because the first switching transistor 31.sub.2 is off. Therefore, an operation current does not flow between the collector and the emitter of the first oscillation transistor 31.sub.1, and an oscillation operation is not achieved.
An oscillation signal (at a frequency in the 900 MHz band) obtained by the second oscillation transistor 32.sub.1 is sent from the emitter thereof to the coupling circuit 33 through the second-oscillation-signal output terminal 36, and then is sent to the common input terminal 37 of the common circuit 34 through the second coupling capacitor 33.sub.4 and the second trap circuit formed of the elements 33.sub.5 and 33.sub.6 in the coupling circuit 33. Then, in the same way as described above, the oscillation signal is amplified by the amplifying transistor 34.sub.1 in the common circuit 34, and the amplified oscillation signal is sent through the signal output terminal 38 to a circuit (not shown) which uses the oscillation signal.
As described above, in the known switch-type oscillating circuit, the first switch signal and the second switch signal, which are complementary, are sent to selectively make the first voltage-controlled oscillating circuit 31 or the second voltage-controlled oscillating circuit 12 operate. The oscillation signal obtained by the operating voltage-controlled oscillating circuit is sent to the common circuit 34 through the coupling circuit 33.
In the known switch-type oscillating circuit, the first and second switch signals, which are complementary, are sent to make one voltage-controlled oscillating circuit, for example, the first voltage-controlled oscillating circuit 31, oscillate, and to make the other voltage-controlled oscillating circuit, for example, the second voltage-controlled oscillating circuit 32, not oscillate. The coupling circuit 33 having the first and second trap circuits provides sufficient isolation between the first-frequency oscillation signal obtained by the first voltage-controlled oscillating circuit 31 and the second-frequency oscillation signal obtained by the second voltage-controlled oscillating circuit 32. Since the first trap circuit is connected in series to the transmission line of the first-frequency oscillation signal and the second trap Circuit is connected in series to the transmission path of the second-frequency oscillation signal, however, the first trap circuit serves as a load for the first-frequency oscillation signal at a path from the input terminal 35 of the first trap circuit through the coupling circuit 33 to the common input terminal 37 of the common circuit 34, and the first-frequency oscillation signal is attenuated. The second trap circuit also serves as a load for the second-frequency oscillation signal at a path from the input terminal 36 of the second trap circuit through the coupling circuit 33 to the common input terminal 37 of the common circuit 34, and the second-frequency oscillation signal is also attenuated.